1. Field of the Invention
The present invention relates to an extensible boom for a crane, and more particularly relates to a method and apparatus for extending and retracting a manual boom section of a multi-section boom.
2. Description of the Prior Art
Cranes having multi-section telescopic booms with hydraulic rams within the booms for selectively extending and retracting the boom sections are, of course, well known in the art. Also, telescopic booms having a manual section and an intermediate section both of which must be extended and retracted by a single hydraulic ram are likewise well known in the art.
For example, Grove U.S. Pat. No. 3,386,594 which issued on June 4, 1968 and Johnston U.S. Pat. No. 3,795,321 which issued on Mar. 5, 1974 each discloses a four section telescopic boom that includes two hydraulic rams. One of the hydraulic rams has its piston rod connected to the inner intermediate power section and its cylinder case selectively connected to the outer powered intermediate section, or to the manual or tip section, for selectively extending and retracting the outer powered intermediate section or the manual section, respectively, relative to the next adjacent supporting boom section.